The present invention is directed to an infrared temperature probe for high pressure use and more specifically to a probe for measuring the temperature of a contained material, such as molten plastic or metal, when it is under high pressure. Typically, such a material is contained within a pressure-resistant, heated enclosure where it is being processed to change its physical properties or to form it into items of useful shape. An example of such processing might be the melting of plastic resin pellets which are then forced under controlled pressure into useful shapes by injection molding or other melt fabrication processes.
During such a process, it is important that the desired temperature be maintained consistently so that the melt will assume the viscosity needed for optimum throughput and for product quality. Too high a temperature is wasteful of energy, among other disadvantages, whereas too low a temperature can reduce throughput besides changing the quality of the finished product. Unwanted temperature variations in a polymer melt at the time of molding can alter the molecular structure and have adverse effects on the tensile strength, flexural strength, impact resistance and optical characteristics of the finished product.
Traditionally, the melt temperature has been measured by inserting a pressure sealed thermocouple probe into the bulkhead containing the melt whereby the melt temperature may be read remotely by means of the thermocouple signals transmitted over suitable wires. One problem with the thermocouple is that the thermal-junction which generates the signal is contained within a protective metallic housing which is in intimate thermal contact not only with the melt but with the bulkhead as well. This means that it measures a temperature which is somewhere between the temperature of the melt and the temperature of the bulkhead. Identical melt temperatures may thus be read with either positive or negative errors depending upon whether, under the circumstances, the bulkhead happens to be cooler than or hotter than the melt. The latter may be the case either when the melt is being allowed to cool or when heating is carried out by a source external to the containing vessel.
One way to avoid erroneous readings is by the use of longer, thinner thermocouple probes whereby the junction is immersed deeper into the melt. The thinner probe reduces heat contact with the bulkhead but also reduces the resistance of the probe to shear forces during plastic flow which often leads to sheared-off probes. Besides temperature errors and fragility, thermocouples are subject to delays in coming to thermal equilibrium with the melt due to the mass of the protective housing. Thus, they register temperature changes relatively slowly compared with other means of temperature measurement.
Another inherently faster method of measuring temperature is an optical one utilizing the infrared radiation which is emitted by hot surfaces in amounts proportional to their temperatures. The measurement of temperatures by means of optical radiation is a well known art and is described extensively in the technical literature. A review of this technique and of its variations is presented by G. A. Hornbeck in Applied Optics, Vol. 5, No. 2, pages 179-186 (Feb. 1966) under the title "Optical Methods of Temperature Measurement." It is also known that infrared radiation can be conducted from the source to an infrared detector via an optical fiber bundle. Such a method is disclosed in U.S. Pat. No. 3,867,697 to Vanzetti et al.